Inspection and Service Practices

10-55 PRESSURE RATIO 34 1215 25 30 20 34 1215 25 30 20 Torquemeter (Turboprop Engines) Only 10 to 15 percent of the thrust produced by a turboprop engine is from propulsive force derived from the jet thrust exiting the exhaust. Engine pressure ratio is not used as an indicator of the power produced by a turboprop engine. Turboprops are usually fitted with a torquemeter that measures torque applied to a shaft turned by the gas generator and power turbines of the turbine engine. The torquemeter can be operated by engine oil pressure metered through a valve that is controlled by a helical ring gear that moves in response to the applied torque. [Figure 10-73] This gear moves against a piston that controls the opening of a valve, which controls the oil pressure flow. This action makes the oil pressure proportional to torque being applied at the propeller shaft.

Generally, transducer is used to transfer the oil pressure into an electrical signal to be read by the flight deck instrument. The read out in the flight deck is normally in lb/ft of torque, or percent horsepower. The torquemeter is very important as it is used to set power settings. This instrument must be calibrated at intervals to assure its accuracy. Tachometer Gas turbine engine speeds are measured by the engines rpm, which are also the compressor/turbine combination rpm of each rotating spool. Most turbofan engines have two or more spools, compressor, and turbine sections that turn independently at different speeds. Tachometers are usually calibrated in percent rpm so that various types of engines can be operated on the same basis of comparison.

[Figure 10-73] Also, turbine speeds are generally very high, and the large numbers of rpm would make it very confusing. Turbofan engines with two spools or separate shafts, high pressure and low pressure spools, are generally referred to as N1 and N2, with each having their own indicator. The main purpose of the tachometer is to be able to monitor rpm under normal conditions, during an engine start, and to indicate an overspeed condition, if one occurs. Exhaust Gas Temperature Indicator (EGT) Exhaust gas temperature (EGT), turbine inlet temperature, (TIT), turbine gas temperature (TGT), interstage turbine temperature (ITT), and turbine outlet temperature (TOT) are all relative temperatures used to monitor the temperature of the exhaust gases entering the first stage turbine inlet guide vanes. Even though these temperatures are taken at different locations on the engine (each engine having one location), they are all relative to the temperature of the gases entering the first stage turbine inlet guide vanes.

Temperature is an engine operating limit and is used to monitor the mechanical integrity of the turbines, as well as to check engine operating conditions. Actually, the temperature of the gases entering the first stage turbine inlet guide vanes is the important consideration, since it is the most critical of all the engine variables. However, it is impractical to measure turbine inlet temperature in most engines, especially large engines. Consequently, temperature thermocouples are inserted at the turbine discharge, where the temperature provides a relative indication of that at the inlet. Although the temperature at this point is much lower than at the inlet, it provides surveillance over the engine’s internal operating conditions. Several thermocouples are usually used, that are spaced at intervals around the perimeter of the engine exhaust duct near the turbine exit. The EGT indicator in the flight deck shows the average temperature measured by the individual thermocouples. [Figure 10-73] Fuel-Flow Indicator Fuel-flow instruments indicate the fuel flow in pounds per hour (lb/hr) from the engine fuel control. Fuel flow in turbine aircraft is measured in lb/hr instead of gallons, because the fuel weight is a major factor in the aerodynamics of large turbine aircraft. Fuel flow is of interest in monitoring fuel consumption and checking engine performance.

[Figure 10-73] Engine Oil Pressure Indicator To guard against engine failure resulting from inadequate lubrication and cooling of the various engine parts, the oil supply to critical areas must be monitored. The oil pressure indicator usually shows the engine oil pump discharge pressure. Engine Oil Temperature Indicator The ability of the engine oil to lubricate and cool depends on the temperature of the oil, as well as the amount of 10-56 OIL PRESS PSI 80 100 0 40 60 20 80 100 0 40 60 20 FUEL FLOW 1 2 3 7 9 12 0 5 1 2 3 7 9 12 0 5 EXT GAS TEMP 5 4 3 2 1 06 7 9 10 8 5 4 3 2 1 06 7 9 10 8 PSI TORQUEMETER 2000 100 50 150 2000 100 50 150 PERCENT RPM 50 40 30 20 100 6070 90 100 80 50 40 30 20 100 6070 90 100 80 °C OIL 5 4 150-50 100 0 50 10-57 oil supplied to the critical areas. An oil inlet temperature indicator frequently is provided to show the temperature of the oil as it enters the oil pressure pump. Oil inlet temperature is also an indication of proper operation of the engine oil cooler.

Turbine Engine Operation The engine operating procedures presented here apply generally to turbofan, turboprop, turboshaft, and auxiliary power units (APU). The procedures, pressures, temperatures, and rpm that follow are intended primarily to serve as a guide. It should be understood that they do not have general application. The manufacturer’s operating instructions should be consulted before attempting to start and operate any turbine engine. A turbofan engine has only one power control lever. Adjusting the power lever, or throttle lever, sets up a thrust condition for which the fuel control meters fuel to the engine. Engines equipped with thrust reversers go into reverse thrust at throttle positions below idle. A separate fuel shutoff lever is usually provided on engines equipped with thrust reversers.

Prior to start, particular attention should be paid to the engine air inlet, the visual condition and free movement of the compressor and turbine assembly, and the parking ramp area fore and aft of the aircraft. The engine is started by using an external air power source, APU, or an already operating engine. Starter types and the engine starting cycle have been discussed previously. On multi-engine aircraft, the engines are usually started by an onboard APU that supplies the air pressure for a pneumatic starter on each engine. Air bled from the APU is used as a source of power for starting the engines.

During the start, it is necessary to monitor the tachometer, the oil pressure, and the exhaust gas temperature. The normal starting sequence is: 1. Rotate the compressor with the starter; 2. Turn the ignition on; and 3. Open the engine fuel valve, either by moving the throttle to idle or by moving a fuel shutoff lever or turning a switch. Adherence to the procedure prescribed for a particular engine is necessary as a safety measure and to avoid a hot or hung start. A successful start is noted first by a rise in exhaust gas temperature. If the engine does not light up, meaning that fuel starts to burn inside of the engine within a prescribed period of time, or if the exhaust gas starting temperature limit is exceeded, a hot start, the starting procedure should be aborted.

Hot starts are not common, but when they do occur, they can usually be stopped in time to avoid excessive temperature by observing the exhaust gas temperature constantly during the start. When necessary, the engine is cleared of trapped fuel or gases by continuing to rotate the compressor with the starter, but with the ignition and fuel turned off. If the engine did not light off during start after the allotted time, about 10 seconds although this time varies from engine to engine, the fuel must be shut off as the engine is being filled with unburned fuel. A hung start is when the engine lights off, but the engine will not accelerate to idle rpm.

Ground Operation Engine Fire Move the fuel shutoff lever to the off position if an engine fire occurs, or if the fire warning light is illuminated during the starting cycle. Continue cranking or motoring the engine until the fire has been expelled from the engine. If the fire persists, CO2 can be discharged into the inlet duct while it is being cranked. Do not discharge CO2 directly into the engine exhaust, because it may damage the engine. If the fire cannot be extinguished, secure all switches and leave the aircraft. If the fire is on the ground under the engine overboard drain, discharge the CO2 on the ground rather than on the engine.

This also is true if the fire is at the tailpipe and the fuel is dripping to the ground and burning. Engine Checks Checking turbofan engines for proper operation consists primarily of simply reading the engine instruments and then comparing the observed values with those known to be correct for any given engine operating condition. After the engine has started, idle rpm has been attained, and the instrument readings have stabilized, the engine should be checked for satisfactory operation at idling speed. The oil pressure indicator, tachometer, and the exhaust gas temperature readings should be compared with the allowable ranges.

Checking Takeoff Thrust Takeoff thrust is checked by adjusting the throttle to obtain a single, predicted reading on the engine pressure ratio indicator in the aircraft. The value for engine pressure ratio, which represents takeoff thrust for the prevailing ambient atmospheric conditions, is calculated from a takeoff thrust setting curve or, on newer aircraft, is a function of the onboard computer. This curve has been computed for static conditions. [Figure 10-74] Therefore, for all precise thrust checking, the aircraft should be stationary, and stable engine operation should be established. If it is needed for calculating thrust during an engine trim check, turbine discharge pressure (Pt7) is also shown on these curves. Appropriate manuals should be consulted for the charts for a specific make and model engine. Engine trimming procedure is also covered in Chapter 2, Engine Fuel & Fuel Metering Systems. The engine pressure ratio computed from the thrust setting curve represents thrust or a lower thrust call part power thrust used for testing. The 10-58 28.00 −40 −20 0 +20 +40 29.00 29.92 31.92 Ambient temperature −°C Engine pressure ratio − P†7/P†2 Turbine discharge pressure − P†7 −IN. Hg Field barometric pressure "Hg Interpolate to locate these points Example: Ambient temperature = 20°C Field barometric pressure = 30.50 "Hg Adjust throttle to obtain EPR determined at point A or the R †7 valve determined at point B Burner pressure limit 24.0025.0026.0027.0028.0029.00 30.00 31.00 A B aircraft throttle is advanced to obtain this predicted reading on the engine pressure ratio indicator, or the part power stop is engaged in the aircraft. If an engine develops the predicted thrust and if all the other engine instruments are reading within their proper ranges, engine operation is considered satisfactory. Full authority digital engine controls (FADEC) (computer controls) also have means of checking the engine with the results displayed on the flight deck.

Ambient Conditions The sensitivity of gas turbine engines to compressor inlet air temperature and pressure necessitates that considerable care be taken to obtain correct values for the prevailing ambient air conditions when computing takeoff thrust. Some things to remember are: 1. The engine senses the air temperature and pressure at the compressor inlet. This is the actual air temperature just above the runway surface. When the aircraft is stationary, the pressure at the compressor inlet is the static field or true barometric pressure, and not the barometric pressure corrected to sea level that is normally reported by airport control towers as the altimeter setting. On FADEC engines, the computer reads this information and sends it to the engine controls.

2. Temperature sensed is the total air temperature (TAT) that is used by several onboard computers. The engine controls set the engine computers according to the TAT. 3. Relative humidity, which affects reciprocating engine power appreciably, has a negligible effect on turbine engine thrust, fuel flow, and rpm. Therefore, relative humidity is not usually considered when computing thrust for takeoff or determining fuel flow and rpm for routine operation. Engine Shutdown On turbine engines that have a thrust reverser, retarding the aircraft throttle to idle or power lever to OFF cuts the fuel supply to the engine and shuts down the engine. On engines equipped with thrust reversers, this is accomplished by means of a separate fuel shutoff lever or switch. When an engine has been operated at high power levels for extended periods of time, a cool down time should be allowed before shutting down. It is recommended the engine be operated at below a low power setting, preferably at idle for a period of 5 minutes to prevent possible seizure of the rotors. This applies, in particular, to prolonged operation at high rpm on the ground, such as during engine trimming. The turbine case and the turbine wheels operate at approximately the same temperature 10-59 when the engine is running. However, the turbine wheels are relatively massive, compared with the case, and are not cooled so readily. The turbine case is exposed to cooling air from both inside and outside the engine. Consequently, the case and the wheels lose their residual heat at different rates after the engine has been shut down. The case, cooling faster, tends to shrink upon the wheels, that are still rotating.

Under extreme conditions, the turbine blades may squeal or seize; thus, a cooling period is required if the engine has been operating at prolonged high speed. Should the turbine wheels seize, no harm normally results, provided no attempt is made to turn the engine over until it has cooled sufficiently to free the wheels. In spite of this, every effort should be made to avoid seizure. To ensure that fuel remains in the lines and that the engine- driven fuel pumps are not starved for fuel that lubricates the pumps, the aircraft fuel boost pump must be turned off after, not before, the throttle or the fuel shutoff lever is placed in the OFF position.

Generally, an engine should not be shut down by the fuel shutoff lever until after the aircraft throttle has been retarded to idle. Because the fuel shutoff valve is located on the fuel control discharge, a shutdown from high thrust settings results in high fuel pressures within the control that can harm the fuel system parts. When an accurate reading of the oil level in the oil tank is needed following an engine shutdown, the engine should be operated and shut down with the oil check taking place within not more than 30 minutes after shutdown. Check the engine manuals for the specific procedure.

Troubleshooting Turbine Engines Included in this section are typical guidelines for locating engine malfunctions on most turbine engines. Since it would be impractical to list all the malfunctions that could occur, only the most common malfunctions are covered. A thorough knowledge of the engine systems, applied with logical reasoning, solves most problems that may occur. encountered. Possible causes and suggested actions are given in the adjacent columns. The malfunctions presented herein are solely for the purpose of illustration and should not be construed to have general application. For exact information about a specific engine model, consult the applicable manufacturer’s instructions.

Turboprop Operation Turboprop engine operation is quite similar to that of a turbojet engine, except for the added feature of a propeller. The starting procedure and the various operational features are very much alike. The turboprop chiefly requires attention to engine operating limits, the throttle or power lever setting, and the torquemeter pressure gauge. Although torquemeters indicate only the power being supplied to the propeller and not the equivalent shaft horsepower, torquemeter pressure is approximately proportional to the total power output and, thus, is used as a measure of engine performance. The torquemeter pressure gauge reading during the takeoff engine check is an important value. It is usually necessary to compute the takeoff power in the same manner as is done for a turbojet engine. This computation is to determine the maximum allowable exhaust gas temperature and the torquemeter pressure that a normally functioning engine should produce for the outside, or ambient, air temperature and barometric pressure prevailing at the time.

Troubleshooting Procedures for Turboprop Engines All test run-ups, inspections, and troubleshooting should be performed in accordance with the applicable engine manufacturer’s instructions. In Figure 10-76 , the troubleshooting procedure for the turboprop reduction gear, torquemeter, and power section are combined because of their inter-relationships. The table includes the principal troubles, together with their probable causes and remedies. Turbine Engine Calibration & Testing Some of the most important factors affecting turbine engine life are EGT, engine cycles (a cycle is generally a takeoff and landing) and engine speed. Excess EGT of a few degrees reduces turbine component life. Low EGT materially reduces turbine engine efficiency and thrust. So, to make the engine highly efficient, the exhaust temperatures need to be as high as possible, while maintaining an EGT operating temperature that does not damage the turbine section of the engine. If the engine is operated at excess exhaust temperatures, engine deterioration occurs. Since the EGT temperature is set by the EGT temperature gauge, it is imperative that it is accurate.

Excessive engine speed can cause premature engine wear and, if extreme, can cause engine failure. One older type of calibration test unit used to analyze the turbine engine is the jetcal analyzer. [Figure 10-77] A jetcal analyzer is a portable instrument made of aluminum, stainless steel, and plastic. The major components of the analyzer are the thermocouple, rpm, EGT indicator, resistance, and insulation check circuits, as well as the potentiometer, temperature regulators, meters, switches, and all the necessary cables, probes, and adapters for performing all tests. Turbine Engine Analyzer Uses Many different types of analyzers are used each with its own function, including onboard systems that use computers to 10-60 Indicated Malfunction Probable Causes Suggested Action • Engine pressure ratio indication has high reading error.

• Engine pressure ratio indication has low reading error due to: - Misaligned or cracked turbine discharge probe. - Leak in turbine discharge pressure line from probe to transmitter. - Inaccurate engine pressure ratio transmitter or indicator. - Carbon particles collected in turbine discharge pressure line or restrictor orifices. • Possible turbine damage and/or loss of turbine efficiency. • If only exhaust gas temperature is high, other parameters normal, the problem may be thermocouple leads or instrument. • Turbine damage. • Damage in compressor section. • Engine-mounted accessory such as constant-speed drive, generator, hydraulic pump, etc.

• Engine main bearings. • Engine bleed-air valve malfunction. • Turbine discharge pressure probe or line to transmitter leaking. • Engine out of trim. Engine has low rpm, exhaust gas temperature, and fuel flow when set to expected engine pressure ratio. Engine has high rpm, exhaust gas temperature, and fuel flow when set to expect engine pressure ratio. Engine has high exhaust gas temperature, low rpm, and high fuel flow at all engine pressure ratio settings. Note: Engines with damage in turbine section may have tendency to hang up during starting. Engine vibrates throughout rpm range, but indicated amplitude reduces as rpm is reduced.

Engine vibrates at high rpm and fuel flow when compared to constant engine pressure ratio. Engine vibrates throughout rpm range, but is more pronounced in cruise or idle rpm range. No change in power setting parameters, but oil temperature high. Engine has higher than normal exhaust gas temperature during takeoff, climb, and cruise. Rpm and fuel flow higher than normal. Engine has high exhaust gas temperature at target engine pressure ratio for takeoff. • Check inlet pressure line from probe to transmitter for leaks. • Check engine pressure ratio transmitter and indicator for accuracy. • Check probe condition.

• Pressure-test turbine discharge pressure line for leaks. • Check engine pressure ratio transmitter and indicator for accuracy. • Confirm indication of turbine damage by: - Checking engine coast-down for abnormal noise and reduced time. - Visually inspect turbine area with strong light. • Re-calibrate exhaust gas temperature instrumentation. • Check turbine as outlined in preceding item. • Check compressor section for damage. • Check each component in turn. • Check scavenge oil filters and magnetic plugs. • Check operation of bleed valve. • Check condition of probe and pressure line to transmitter.

• Check engine with jetcal. Re-trim as desired. 10-61 Indicated Malfunction Probable Causes Suggested Action • Pressurizing and drain valve malfunction. • Cracked air duct. • Fuel control malfunction. • Subzero ambient temperatures. • Compressor section damage. • Turbine section damage. • Scavenge pump failure. • Fuel heater malfunction. • Scavenge pump failure. • High sump pressure. • Gearbox seal leakage. • Can be caused by high airflow through the tank, foaming oil, or unusual amounts of oil returned to the tank through the vent system. Engine rumbles during starting and at low power cruise conditions.

Engine rpm hangs up during starting. High oil temperature. High oil consumption. Overboard oil loss. • Replace pressurizing and drain valves. • Repair or replace duct. • Replace fuel control. • If hang-up is due to low ambient temperature, engine usually can be started by turning on fuel booster pump or by positioning start lever to run earlier in the starting cycle. • Check compressor for damage. • Inspect turbine for damage. • Check lubricating system and scavenge pumps. • Replace fuel heater. • Check scavenge pumps. • Check sump pressure as outlined in manufacturer’s maintenance manual. • Check gearbox seal by pressurizing overboard vent.

• Check oil for foaming. • Vacuum-check sumps. • Check scavenge pumps. test aircraft systems. Depending upon the specific analyzer used, procedures vary somewhat, but the basic test are outlined here. Always refer to the specific instructions associated with the analyzer being used. Most analyzers may be used to: 1. Functionally check the aircraft EGT system for error, without running the engine or disconnecting the wiring. 2. Check individual thermocouples before placement in a parallel harness. 3. Check each engine thermocouple in a parallel harness for continuity. 4. Check the thermocouples and parallel harness for accuracy.

5. Check the resistance of the EGT circuit. 6. Check the insulation of the EGT circuit for shorts to ground, or for shorts between leads. 7. Check EGT indicators , either in or out of the aircraft, for error. 8. Determine engine rpm accuracy during engine testing. Added to this is the checking and troubleshooting of the aircraft tachometer system. 9. Establish the proper relationship between the EGT and engine rpm during engine run-up. Analyzer Safety Precautions Observe the following safety precautions while operating the engine analyzer or other types of test equipment: 1. Never use a voltammeter to check the potentiometer for continuity. If a voltammeter is used, damage to the galvanometer and standard battery cell results.

2. Check the thermocouple harness before engine run-up. This must be done because the circuit must be correct before the thermocouples can be used for true EGT pickup. 3. For safety, ground the jetcal analyzer when using an AC power supply. Any electrical equipment operated on AC power and utilizing wire-wound coils, such as the probes with the jetcal analyzer, has an induced voltage on the case that can be discharged if the equipment is not grounded. This condition is not apparent during dry weather, but on damp days the operator can be shocked slightly. Therefore, for the operator’s protection, the jetcal analyzer should be grounded using the pigtail lead in the power inlet cable.

4. Use heater probes designed for use on the engine 10-62 Trouble Probable causes Remedy • No air to starter. • Propeller brake locked. • Starter speed low because of inadequate air supply to starter. • If fuel is not observed leaving the exhaust pipe during start, fuel selector valve may be inoperative because of low power supply or may be locked in “OFF.” • Fuel pump inoperative. • Aircraft fuel filter dirty. • Fuel control cutoff valve closed. • Insufficient fuel supply to control unit. • Fuel control main metering valve sticking. • Fuel control bypass valve sticking open. • Drain valve stuck open. Starting fuel enrichment pressure switch setting too high.

• Fuel control bypass valve sticking closed. • Fuel control acceleration cam incorrectly adjusted. • Defective fuel nozzle. • Fuel control thermostat failure. • Acceleration cam of fuel control incorrectly adjusted. • Unstable fuel control governor operation. • Oil supply low. • Oil pressure transmitter or indicator giving false indication. • Seal failure. • Faulty propeller governor. • Faulty fuel control or air sensing tip. • Vibration pickup or vibration meter malfunction. Power unit fails to turn over during attempted start. Power unit fails to start. Engine fires, but will not accelerate to correct speed.

Acceleration temperature too high during starting. Acceleration temperature during starting too low. Engine speed cycles after start. Power unit oil pressure drops off severely. Oil leakage at accessory drive seals. Engine unable to reach maximum controlled speed of 100 percent. Vibration indication high. • Check started air valve solenoid and air supply. • Unlock brake by turning propeller by hand in direction of normal rotation. • Check starter air valve solenoid and air supply. • Check power supply or electrically operated valves. Replace valves if defective. • Check pump for sheared drives or internal damage. Check for air leaks at outlet.

• Clean filter and replace filtering elements if necessary. • Check electrical circuit to ensure that actuator is being energized. Replace actuator or control. • Check fuel system to ensure all valves are open and pumps are operative. • Flush system. Replace control. • Replace drain valve. Replace pressure switch. • Flush system. Replace control. • Replace control. • Replace nozzle with a known satisfactory unit. • Replace control. • Continue engine operation to allow control to condition itself. • Check oil supply and refill as necessary. • Check transmitter or indicator and repair or replace if necessary.

• Replace seal or seals. • Replace propeller control assembly. • Replace faulty control. If dirty, use air pressure in reverse direction of normal flow through internal engine passage and sensing tip. • Calibrate vibration meter. • Start engine and increase power gradually. • Observe vibration indicator. If indications prove pickup to be at fault, replace it. If high vibration remains as originally observed, remove power unit for overhaul. 10-63 50 40 60 30 70 20 80 10 90 0 100 GALVO-2 GALVO-3 RPM SW-9 MECH ZERO OFF RANGE MECH ZERO SELECTOR SWITCH RANGE ELEC ZERG LEG ZERO R - 3 R 10 R 1 R 100 110V 60-400 RPM SW-4 REG INSUL HEATER CABLE CHECK CABLE SW-3 EGT IND CHECK RESISTANCE CHECK THERMOCOUPLE CHECK ADJ RHEO INPUT P-3 S-1 S-2 R-1 2 AMP 20 AMP R 1000 R-2 SW-9 SW-1 EGT T/C RES OFF INSUL TEMP REGULATOR BATTERY OPERATION RPM CHECK POTENTIOMETER INSULATION CHECK RPM check (takeal unit) Heater cable receptacle (S-1) Check cable receptacle (S-2) Potentiometer Insulation check meter operating knob Power input receptacle (P-1) Insulation check meter Temperature regulator Fuses Heater cable switch (SW-2) Resistance insulation check input receptacle (P-2) Resistance check selector switch Tachometer -EGT indicator check input receptacle (P-3) EGT indicator selector switch (SW-4) EGT indicator adjusting rheostat Selector switch (SW-1) thermocouples to be tested. Temperature gradients are very critical in the design of heater probes. Each type of aircraft thermocouple has its own specially designed probe. Never attempt to modify heater probes to test other types of thermocouples.

5. Do not leave heater probe assemblies in the exhaust nozzle during engine run-up. 6. Never allow the heater probes to go over 900 °C (1,652 °F). Exceeding these temperatures results in damage to the jetcal analyzer and heater probe assemblies. Continuity Check of Aircraft EGT Circuit To eliminate any error caused by one or more inoperative aircraft thermocouples, a continuity check is performed. The check is made by heating one heater probe to between 500 and 700 °C and placing the hot probe over each of the aircraft thermocouples, one at a time. The EGT indicator must show a temperature rise as each thermocouple is checked.

When large numbers (eight or more) of thermocouples are used in the harness, it is difficult to see a rise on the aircraft instrument because of the electrical characteristics of a parallel circuit. Therefore, the temperature indication of the aircraft thermocouples is read on the potentiometer of the analyzer by using the check cable and necessary adapter. Functional Check of Aircraft EGT Circuit During the EGT system functional test and the thermocouple harness checks, the analyzer has a specific degree of accuracy at the test temperature, which is usually the maximum operating temperature of the turbine engine. [Figure 10-78] Each engine has its own maximum operating temperature, that can be found in applicable technical instructions.

The test is made by heating the engine thermocouples in the exhaust nozzle or turbine section to the engine test temperature. The heat is supplied by heater probes through the necessary cables. With the engine thermocouples hot, their temperature is registered on the aircraft EGT indicator. At the same time, the thermocouples embedded in the heater probes, which are completely isolated from the aircraft system, are picking up and registering the same temperature on the test analyzer. The temperature registered on the aircraft EGT indicator should be within the specified tolerance of the aircraft system and the temperature reading on the temperature analyzer.

When the temperature difference exceeds the allowable tolerance, troubleshoot the aircraft system. 10-64 EGT Indicator Check The EGT indicator is tested after being removed from the aircraft instrument panel and disconnected from the aircraft EGT circuit leads. Attach the instrument cable and EGT indicator adapter leads to the indicator terminals and place the indicator in its normal operating position. Adjust the analyzer switches to the proper settings. The indicator reading should correspond to the readings of the analyzer within the allowable limits of the EGT indicator. Correction for ambient temperature is not required for this test, as both the EGT indicator and analyzer are temperature compensated. The temperature registered on the aircraft EGT indicator should be within the specified tolerance of the aircraft system and the temperature reading on the analyzer readout. When the temperature difference exceeds the allowable tolerance, troubleshoot the aircraft system.

Resistance & Insulation Check The thermocouple harness continuity is checked while the EGT system is being checked functionally. The resistance of the thermocouple harness is held to very close tolerances, since a change in resistance changes the amount of current flow in the circuit. A change of resistance gives erroneous temperature readings. The resistance and insulation check circuits make it possible to analyze and isolate any error in the aircraft system. How the resistance and insulation circuits are used is discussed with troubleshooting procedures. Tachometer Check To read engine speed with an accuracy of ±0.1 percent during engine run, the frequency of the tachometer-generator (older style) is measured by the rpm check analyzer. The scale of the rpm check circuit is calibrated in percent rpm to correspond to the aircraft tachometer indicator, which also reads in percent rpm. The aircraft tachometer and the rpm check circuit are connected in parallel, and both are indicating during engine run-up. The rpm check circuit readings can be compared with the readings of the aircraft tachometer to determine the accuracy of the aircraft instrument.

Many newer engines use a magnetic pickup that counts passing gear teeth edges, which are seen electrically as pulses of electrical power as they pass by the pickup. [Figure 10-79] By counting the amount of pulses, the rpm of the shaft is obtained. This type of system requires little maintenance, other than setting the clearance between the gear teeth and the magnetic pickup. Troubleshooting EGT System An appropriate analyzer is used to test and troubleshoot the aircraft thermocouple system at the first indication of trouble, or during periodic maintenance checks. The test circuits of the analyzer make it possible to isolate the troubles listed below. Following the list is a discussion of each trouble mentioned.

1. One or more inoperative thermocouples in engine parallel harness. 2. Engine thermocouples out of calibration. 3. EGT indicator error. 4. Resistance of circuit out of tolerance. 5. Shorts to ground. 6. Shorts between leads. One or More Inoperative Thermocouples in Engine Parallel Harness This error is found in the regular testing of aircraft thermocouples with a hot heater probe and is a broken lead wire in the parallel harness, or a short to ground in the harness. In the latter case, the current from the grounded thermocouple can leak off and never be shown on the indicator. However, this grounded condition can be found by using the insulation resistance check.

Engine Thermocouples Out of Calibration When thermocouples are subjected for a period of time to oxidizing atmospheres, such as encountered in turbine engines, they drift appreciably from their original calibration. On engine parallel harnesses, when individual thermocouples can be removed, these thermocouples can be bench-checked, using one heater probe. The temperature reading obtained from the thermocouples should be within manufacturer’s tolerances. 10-65 EGT Circuit Error This error is found by using the EGT and comparing the reading of the aircraft EGT indicator with the analyzer temperature reading. [Figure 10-78] The analyzer and aircraft temperature readings are then compared.

Resistance of Circuit Out of Tolerance The engine thermocouple circuit resistance is a very important adjustment since a high-resistance condition gives a low indication on the aircraft EGT indicator. This condition is dangerous, because the engine is operating with excess temperature, but the high resistance makes the indicator read low. It is important to check and correct this condition. Shorts to Ground/Shorts Between Leads These errors are found by doing the insulation check using an ohmmeter. Resistance values from zero to 550,000 ohms can be read on the insulation check ohmmeter by selecting the proper range.

Troubleshooting Aircraft Tachometer System A function of the rpm check is troubleshooting the aircraft tachometer system. The rpm check circuit in the analyzer is used to read engine speed during engine run-up with an accuracy of ±0.1 percent. The connections for the rpm check are the instrument cable and aircraft tachometer system lead to the tachometer indicator. After the connections have been made between the analyzer rpm check circuit and the aircraft tachometer circuit, the two circuits, now classed as one, are a parallel circuit. The engine is then run-up as prescribed in applicable technical instructions. Both systems can be read simultaneously.

If the difference between the readings of the aircraft tachometer indicator and the analyzer rpm check circuit exceeds the tolerance prescribed in applicable technical instructions, the engine must be stopped, and the trouble located and corrected.